Prior to the development of the dual permanent magnet generator (PMG), aircraft typically used wound field generating systems as a power supply. With weight and size considerations becoming of increased importance, however, PMG devices of the type shown in U.S. Pat. Nos. 4,305,031 and 4,654,577 were developed. These generators were considered relatively small and lightweight devices for generating electrical power from a motive power source because they do not require a separate electrical source for field excitation.
A PMG does not require a complex cooling apparatus and eliminates the need for a convertor by driving the system with a constant speed drive to limit the maximum voltage excursion in the case of a system fault. The output voltage of a PMG is, however, a function of the relative speed between the rotor and stator and of the current drawn by a load. This drawback has prevented PMG's from being used as a main power source in aircraft in lieu of wound field machines having exciters and rotating rectifiers driven at high speed. These high speeds required complex cooling apparatus to dissipate heat developed on the windings and in the rotating rectifiers.
When used in a generator, there is a set voltage on the generator which varies as the load varies and which cannot be controlled electronically. It has been found that the rotors in PMGs create a voltage control problem as the load varies. A dual permanent magnetic generator concept controls voltage by shifting two high speed rotors in and out of phase to keep the voltage constant as the load varies. For example, at a two per unit load, the magnetic poles are aligned, whereas at no load, the magnetic poles would be 78.degree. out of phase. If the main high speed rotors of the PMG which, at no load, are out phase with each other by about 78.degree. were kept in the same phase relationship as the generator load is increased, the output voltage would droop.
A PMG system using left-hand and right-hand helical ball splines to effect adjustment of rotors relative to each other is disclosed in commonly assigned U.S. Pat. No. 4,879,484. This system comprises a stator having an output winding for producing an output voltage, a rotor within the stator, the rotor including first and second magnet rotors rotatable about a common axis and a mechanism for angularly adjusting the first and second rotors relative to one another about the common axis. The angular adjustment of the rotors provides for rotation of each of the first and second rotors in opposite directions relative to the other for causing a relative angular adjustment of the rotors. Rotating the rotors simultaneously in opposite directions, each by half the required angle n requires much less power to overcome inertia than rotating only one rotor the full angle. A common drive shaft extending along the common axis of the rotors is used for rotatably driving both the first and second rotors about the common axis.
Left- and right-hand helical ball splines are located between the driven shaft and the first and second rotors, respectively. Rotation of the first and second rotors in opposite directions relative to one another is accomplished by causing axial movement of the drive shaft along the common axis relative to the rotors. In the system of the aforementioned patent, bearings are provided for rotatably supporting the rotors at essentially fixed locations at their ends along the common axis, and the drive shaft is permitted to move axially along the common axis relative to the rotors for angular adjustment of the rotors. The means for axially moving the drive shaft comprises a hydraulic drive means or an electric motor. In either case, the use of the oppositely directed helical ball splines together with the axial movement of the single drive shaft, reduces the friction required for angular adjustment of the rotors and thereby reduces the power necessary for accomplishing the adjustment.
One problem encountered with a dual PMG of the aforementioned type concerns the need to support and oil cool the center rotor bearings, that is, those bearings rotatably supporting the adjacent ends of the two in-line rotors of the generator. Electrical losses should be avoided or kept to a minimum as part of a solution to this problem.
U.S. Pat. No. 2,476,892 discloses a twin-motor bearing housing construction having a central bearing-housing which has a lubricating pipe. U.S. Pat. No. 4,728,840 depicts a water-cooled AC and DC motor-generator set on a common shaft with series cooling flow path. U.S. Pat. No. 4,514,652 relates to a liquid cooled high speed synchronous machine wherein oil flows axially through the stator in a single rotary generator. U.S. Pat. No. 4,644,207 is directed to an integrated dual pump system having center bearing separating two rotors. The bearings are supplied with lubricant through passages that extend radially into the bearings.